The present invention relates to a sewing machine having a motor and an armshaft which are coupled together by means of a pulley-and-belt arrangement.
Conventional industrial sewing machines comprise a foot pedal, a speed setting device responsive to the amount of depression of the pedal to establish a reference speed, and an armshaft with a needle driven by a motor. A pulley-and-belt arrangement is employed to couple the motor to the armshaft to cause reciprocation of the needle. A speed control circuit detects the difference between the actual speed of the motor sensed by a tachogenerator and the reference speed and controls the motor so that the difference reduces to zero. The speed of rotation of the armshaft depends on the ratio between the diameter of the motor pulley and the diameter of the armshaft pulley. For this reason, it has been necessary to calibrate the armshaft speed by measuring the diameter of the armshaft pulley and selecting a motor pulley corresponding to the measured diameter. However, armshaft pulleys vary in size between different sewing machines. The calibration is thus required for each sewing machine, which is tedious and time consuming for mass production of industrial sewing machines.